(1) Field of the Invention
The present invention relates to a chromatic dispersion compensator which compensates chromatic dispersion accumulated in a light signal propagating through an optical fiber transmission line in a field of optical communication, particularly to the chromatic dispersion compensator which generates the variable chromatic dispersion by utilizing an optical component provided with a function of demultiplexing an input light according to a wavelength.
(2) Related Art
For example, the conventional chromatic dispersion compensator includes one in which a so-called Virtually Imaged Phased Array (VIPA) is utilized (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2000-511655 and JP-A No. 2002-514323). VIPA demultiplexes a Wavelength Division Multiplexing (WDM) light into plural light fluxes, which can spatially be identified according to the wavelengths.
FIG. 8 is a perspective view showing a configuration example of the conventional VIPA type chromatic dispersion compensator. FIG. 9 is a top view of the configuration example shown in FIG. 8.
As shown in the drawings, in the conventional VIPA type chromatic dispersion compensator, for example, after an emitted light from one end of optical fiber 130 through an optical circulator 120 is converted into parallel light with a collimating lens 140, the light is focused onto one line segment with a line focusing lens 150, and the light is incident to a space between parallel planes opposing each other through a light entrance window 116 of a VIPA plate 110. The light incident to the VIPA plate 110 is repeatedly multiply reflected between a reflection multilayer film 112 and a reflection multilayer film 114. The reflection multilayer film 112 is formed on one of planes of the VIPA plate 110, and the reflection multilayer film 112 has reflectance lower than 100%. The reflection multilayer film 114 is formed on the other plane, and the reflection multilayer film 114 has the reflectance of about 100%. At this point, the several-percent of light is transmitted through the reflection plane and emitted to the outside of the VIPA plate 110 in each reflection on the surface of the reflection multilayer film 112.
The lights transmitted through the VIPA plate 110 interfere with one another to form plural light fluxes having the different traveling directions according to the wavelengths. As a result, when the light fluxes are focused on one point with a lens 160, the focal position of each light flux is moved on a straight line according to a change in wavelength. The lights, emitted from the VIPA plate 110 and focused with the lens 160, are reflected at different positions on a three-dimensional mirror 170 according to the wavelengths and returned to the VIPA plate 110 by arranging the three-dimensional mirror 170. The lights reflected from the three-dimensional mirror 170 travel in the different directions depending on the wavelengths, and optical paths of the lights are shifted when the lights are returned to the VIPA plate 110. The different wavelength components propagate through different distances by changing the optical path shift amount according to the wavelengths, which performs the chromatic dispersion compensation of the input light.
Thus, when a model shown in FIG. 10 is considered, behavior of the light which is multiply reflected with the VIPA plate 110 is similar to the light in a well-known Echelon grating which is of a step-shaped diffraction grating. Therefore, it can be considered that the VIPA plate 110 is a virtual diffraction grating. In consideration of interference conditions at the VIPA plate 110, as shown in right side of FIG. 10, upper sides of the emitted lights interfere on the condition of a short wavelength based on an optical axis and lower sides interfere on the condition of a long wavelength, so that short wavelength components of the light signals having the wavelengths are emitted onto the upper side and long wavelength components are emitted onto the lower side. The conventional VIPA type chromatic dispersion compensator has advantages in that the chromatic dispersion can be compensated over a wide range, the wavelength (transmission wavelength) of the light signal to be compensated can be changed by adjusting of the VIPA plate 110 to shift a transmission band of the periodically-generated light in a wavelength axis direction, and the like.
Further, for the conventional VIPA type chromatic dispersion compensator, for example, there is also known a technology in which flattening of the transmission band of the light is performed by utilizing a spatial filter having a two-dimensionally variable transmission loss property or by two-dimensionally changing the reflectance of the reflection plane of the three-dimensional mirror 170 (for example, see JP-A No. 2003-207618). There is also proposed a technology in which flattening of the transmission band is achieved by changing an angle of the three-dimensional mirror 170 according to the wavelength to change reflection efficiency (for example, see JP-A No. 2003-294999).
In the conventional VIPA type chromatic dispersion compensator, sometimes it is desired that the larger chromatic dispersion can be compensated. However, in principle, the VIPA type chromatic dispersion compensator has a characteristic that, when a chromatic dispersion compensation amount (absolute value) is increased, widths of the transmission bands periodically generated are decreased and transmission light loss is increased, so that there is a problem that the compensable chromatic dispersion amount is restricted.
The reason why the increase in chromatic dispersion compensation amount decreases the transmission bandwidth in the VIPA type chromatic dispersion compensator will briefly be described. For example, as shown in FIG. 11, when the light signal having a central wavelength λC is incident to a light entrance window 116 of the VIPA plate 110, the component having the central wavelength λC of the light multiply reflected between the parallel planes wavelength λC is emitted from the VIPA plate 110 according to an intensity distribution I1 in which intensity is attenuated as the number of the multiple reflection times is increased. Then, the light having the central wavelength λC is reflected by the three-dimensional mirror 170 through the convergent lens 160 and returned to the VIPA plate 110. The light having a central wavelength λC is incident to the VIPA plate 110 again while having an intensity distribution I2 which is symmetrical to the intensity distribution I1 of the emitted light, and the light is multiply reflected. Then, the light is emitted from the light entrance window 116. At this point, the intensity of the light (transmission light) having the central wavelength λC emitted from the light entrance window 116 can conceptually be expressed by a shaded area where the intensity distributions I1 and I2 overlap each other.
As shown in FIG. 12, during negative dispersion compensation, in the light on a short wavelength λS side included in the light signal, an intensity distribution I2′ of the light returned to the VIPA plate 110 is shifted upward as described in FIG. 12 with respect to the light of the central wavelength λC according to a reflection position at the three-dimensional mirror 170, so that the area where the intensity distributions I1 and I2′ overlap each other is decreased. Therefore, the transmittance of the light on the short wavelength λs side is decreased, i.e., loss is increased.
As shown in FIG. 13, during the negative dispersion compensation, in the light on a long wavelength λL side included in the light signal, an intensity distribution I2″ of the light returned to the VIPA plate 110 is shifted downward in FIG. 13 however the shift amount is small compared with the short wavelength λS side, so that the transmittance is decreased. When the chromatic dispersion compensation amount (absolute value) is increased, because the shift amount upward the short wavelength λS side and the shift amount downward the long wavelength λL side are increased respectively, the transmittance on the short wavelength λS side with respect to the central wavelength λC and the transmittance on the long wavelength λL side are decreased (the loss is increased). Accordingly, as shown in the upper portion of FIG. 14, as the chromatic dispersion compensation amount (absolute value) is increased, the transmission bandwidth is decreased. A lower portion of FIG. 14 illustrates a relationship between the wavelength and group delay time when the chromatic dispersion compensation amount is varied.
In the configuration of the conventional VIPA type chromatic dispersion compensator, examples of the method of increasing the chromatic dispersion compensation amount include the method of lengthening a distance between the parallel planes of the VIPA plate 110, the method of decreasing an inclination angle of the VIPA plate 110 with respect to the incident light, and the method of increasing a curvature of the reflection plane of the three-dimensional mirror 170.
However, because the distance between the parallel planes of the VIPA plate 110 determines a period (Free Spectral Range (FSR)) in which the transmission band is repeated at constant wavelength (frequency) intervals, it is necessary that the distance between the parallel planes is set at a value corresponding to a wavelength interval (channel interval) of the light signal included in the WDM light to be compensated. It is necessary that the inclination angle of the VIPA plate 110 is set such a value that the light does not go out the VIPA plate 110 through the light entrance window 116 after the light incident from the light entrance window 116 is reflected by the opposing surface 112. Therefore, it is necessary to secure the inclination angle not lower than a predetermined angle, which is determined according to a beam diameter of the incident light, focused by the line focusing lens 150. It is difficult to produce the three-dimensional mirror 170 having the large curvature, and the remarkable decrease in transmission bandwidth occurs in the three-dimensional mirror 170 having the large curvature.
Accordingly, in order to realize the VIPA type chromatic dispersion compensator, which can compensate the larger chromatic dispersion, it is necessary that design is performed in sufficient consideration of the various constraint conditions unique to the configuration of the above VIPA type chromatic dispersion compensator.
Separately from the problem concerning the increase in chromatic dispersion compensation amount, the conventional VIPA type chromatic dispersion compensator has also the following problem concerning temperature control of the VIPA plate.
In the conventional VIPA type chromatic dispersion compensator, practically, in order that the wavelength band of each channel included in the WDM light to be compensated is included in the transmission band periodically generated, the periodic transmission band is optimized by controlling a temperature of the VIPA plate 110 to change the optical path length. Usually, as shown in FIG. 15, for the transmission band corresponding to one channel in the periodic transmission band, the temperature of the VIPA plate 110 is controlled such that a 3-dB central wavelength coincides with the central wavelength of the light signal. The 3-dB central wavelength is the central wavelength when a range in which the transmittance is decreased from the maximum value by 3 dB is set at the transmission band.
As shown in FIG. 14, in the conventional VIPA type chromatic dispersion compensator, since the spectral shape of the transmission band is largely changed when the chromatic dispersion compensation amount is changed, in each setting change of the chromatic dispersion. compensation amount, it is necessary that the transmission band is adjusted by controlling the temperature of the VIPA plate 110
Because the reflection position of the light is changed by moving the three-dimensional mirror 170, so that it takes a relatively short time to change the setting of the chromatic dispersion compensation amount. On the other hand, in the temperature control of the VIPA plate 110, because optical glass used for the VIPA plate has a small temperature coefficient of a refractive index of (for example, the temperature coefficient of the refractive index of BK7 which is of the typical optical glass is 2.2×10−6 (1/° C.)), there is a problem that a long time is required to adjust the wavelength in association with the setting change of the chromatic dispersion compensation amount.